Prior proposals and practices for lining crucible and the like type furnaces fall generally into two categories one of which utilizes preformed fire brick and the other castable or hydraulic cement. If fire bricks are used, these must be assembled within the chamber and suitably sealed to one another as by mortar. If hydraulic cement material is used, the chamber must be provided with a suitable form spaced inwardly from the chamber wall and into which freshly mixed hydraulic cementing material is poured. After the removal of the forms the cement is allowed to dry for an extended period after which it must be slowly heated at a safe rate permitting the remaining water to escape to the atmosphere without causing an explosion. Both prior types are objectionably heavy, have a high heat storage capacity, and objectionable high heat conductivity.